1. Field of the Invention
The invention relates generally to the design of ophthalmic contact lenses and more precisely to a method of determining the shape of an ophthalmic contact lens capable of correcting optical aberrations of the eye beyond defocusing or astigmatism. It also relates to a system for implementing the method.
2. Description of the Prior Art
Existing ophthalmic contact lenses, despite all the advances in their design, merely correct the coarsest optical defects of the eye, namely defocusing and astigmatism. It has long been known in the art that the eye can suffer from additional defects resulting from higher order optical aberrations, of a more refined nature, affecting the various components of the eye: cornea, lens or intraocular media. These higher order optical aberrations of the eye disrupt the image formed on the retina, which interfere with vision even after any defocusing or astigmatism defects have been corrected. In some pathological cases of irregular corneas, for example keratoconus, these aberrations make it practically impossible to see shapes.
Ophthalmic measuring techniques have recently been developed for accurately measuring higher order optical aberrations of the eye, in addition to the standard defects of defocusing and astigmatism. For example, U.S. Pat. No. 5,777,719 proposes a method and system for accurately measuring optical aberrations of the eye by analyzing a laser wavefront reflected by the retina of the eye using the Hartmann-Shack method. It is theoretically possible to determine from this measurement the shape of an ophthalmic contact lens for compensating the measured aberrations. This technique personalizes the lens to the particular optical aberrations of the eye of the patient.
Thought has therefore been given to a method of determining the shape of an ophthalmic contact lens for correcting optical aberrations of the eye including a step of measuring the optical aberrations of the eye to be corrected and a step of determining the shape of said lens to correct those aberrations from the measured optical aberrations of the eye to be corrected.
The front face of a contact lens is the convex face of the lens, which is on the opposite side of the lens to the eye, and the rear face of a lens is the concave face of the lens, which is in contact with the eye.
However, at this stage it was realized that a lens designed on the basis of the above data alone does not provide the hoped-for compensation when it is installed on the eye of the patient.
The lenses generally have a rear face of simple shape in which only the central radius of curvature is matched to the surface of the cornea. The cornea usually has a complex shape that varies from one person to another. The difference in shape between the cornea and the rear face of the lens produces several effects. It causes deformation of the lens and creates an irregular thickness film of tears between the lens and the cornea. These two effects induce additional unwanted aberrations that degrade visual performance.
Another form of personalization of ophthalmic contact lenses is known in the art. It is known in the art that the surface of the cornea is not perfectly regular but, to the contrary, is generally asymmetrical and aspherical. Depending on its degree and its nature, this surface irregularity of the cornea can be a source of conflict between the surface and the rear surface of the contact lens, which is generally perfectly regular, spherical, circular aspherical or toroidal. This is uncomfortable for the wearer, sometimes leading to irritation of the cornea and rejection of the lens, forcing the patient to use eyeglasses in preference to contact lenses. Making contact lenses whose rear face is xe2x80x9cadaptedxe2x80x9d to the irregular surface of the cornea is one proposal for solving this problem, for example in U.S. Pat. No. 5,570,142. The xe2x80x9cadaptationxe2x80x9d consists of a corresponding relationship of shape between the posterior surface of the lens and the surface of the cornea, which can be complete, over all of the posterior surface of the lens, or partial, localized at the periphery of the lens, to provide a stable and regular seating for the lens on the cornea.
However, regardless of the manner in which the posterior surface of the lens is adapted to the surface of the cornea, personalized adaptation merely improves the comfort of the wearer or prevents rotation of the lens on the eye, and does not take any account of ocular aberrations.
In the light of the above information, the object of the invention is to conceive a method of determining the shape of a personalized ophthalmic contact lens which is capable, when installed on the eye of the patient, of correcting the higher order aberrations measured on the eye concerned by limiting the generation of unwanted aberrations.
The invention proposes a method of determining the shape of an ophthalmic contact lens for correcting optical aberrations of an eye, which method includes the steps of:
measuring the optical aberrations of the eye to be corrected,
determining the shape of the front face of said lens from the measured optical aberrations of the eye to be corrected in order to correct said aberrations,
measuring the topography of the cornea of the eye to be corrected, and
determining the shape of the rear face of the lens from the measured topography of the cornea in order to limit the generation of unwanted aberrations when the lens is placed on the eye,
and in which method, to determine the shape of the front face of the lens, data relating to the shape determined for the rear face of the lens is combined with the measured optical aberrations of the eye.
Knowing the wavefront to be corrected, the shape of the rear face, the wavelength, the index of the material and the thickness at the center, the required shape of the front face can be calculated by solving the converse problem of three-dimensional ray tracing.
The above method takes into account a constraint of fundamental practical importance previously unidentified in the context of correcting higher order optical aberrations of the eye. To correct higher order aberrations of the eye by means of a contact lens it is not sufficient to measure the aberrations and to design the shape of the front face of the lens so that it corrects those aberrations. It is also necessary to avoid, or at least to control, in order to take them into account, the deformations of the lens on the eye and the irregular thickness of the film of tears between the cornea and the lens. The fact that the rear face of the lens designed by the method of the invention is personalized provides total control over these two essential parameters, namely the deformation of the lens and the thickness of the layer of tears, on the basis of the measured topography of the cornea. Clearly these two parameters condition the effective optical correction achieved by the lens. Controlling them is therefore essential in the context of correcting higher order aberrations of the eye in that such correction is by its very nature much more refined than merely correcting defocusing or astigmatism, and consequently requires high precision in the shape of the lens, not only in vitro, that is to say during manufacture, but also in vivo, that is to say when it is in place on the eye of the patient.
The mechanical behavior of lens in vivo is therefore controlled by a particular design of the rear face of the lens. The particular shape of that face must be taken into account in designing the front face of the lens, which in the final analysis determines the global optical correction applied by the lens. The shape of the rear face of the lens conditions the optical characteristics of the lens itself and also the thickness of the film of tears, which may be irregular (but controlled), and which has a decisive effect on the global optical correction obtained.
The shape of the rear face of the lens can be designed so that there is a film of tears of constant thickness between the lens and the cornea. In another embodiment, this face is designed so that the thickness of the film of tears increases with the distance away from the center of the lens.
The topography of the cornea of the eye to be corrected and the aberrations of said eye to be corrected are preferably measured in a common spatial frame of reference. To be more precise, the common spatial frame of reference comprises a first axis coincident with the main line of vision of the eye to be corrected, a horizontal second axis orthogonal to the first axis and a third axis orthogonal to the first and second axes. The main line of vision corresponds to the line passing through the fixing point in the instrument and the center of the pupil of the eye.
In accordance with another advantageous aspect of the invention, to improve the optical correction further, in particular for certain patients requiring increased precision, the method can further include the steps of:
fabricating a test lens having front and rear faces conforming to those determined by the aforementioned steps,
measuring in vivo the optical aberrations of the optical system comprising the eye to be corrected fitted with the fabricated test lens, and
correcting the shape of the front face of the lens on the basis of data relating to the initially determined shape of the test lens and the new measured optical aberrations of the eye fitted with the test lens.
The invention also provides a system for implementing the above method, the system including:
a measuring unit for measuring the topography of the cornea of the eye to be corrected and delivering digital data representative of that topography,
a measuring unit for measuring the aberrations of the eye to be corrected and delivering digital data representative of those aberrations, and
an electronic calculator unit adapted to determine the shapes of the front and rear faces of the lens from data supplied to it by the measurement unit for measuring the topography of the cornea of the eye to be corrected and the measurement unit for measuring the aberrations of said eye to be corrected and to deliver digital data representative of those shapes.
According to another advantageous aspect of the invention, the method can be simplified in certain specific pathological cases by having the step of measuring the aberrations of the eye to be corrected coincide with the step of measuring the topography of the cornea of said eye to be corrected, the aberrations being deduced by calculating the measured topography of said cornea.
The above type of method can be advantageous if the aberrations of the cornea are dominant, which has already been observed.
In this case, the system for implementing the above method includes:
a measuring unit for measuring the topography of the cornea of the eye to be corrected and delivering digital data representative of that topography, and
an electronic calculator unit adapted to estimate the aberrations produced by the cornea from the data supplied by the measurement unit for measuring the topography of the cornea and to determine the shapes of the front and rear faces of the lens from data supplied to it by the measurement unit for measuring the topography of the cornea and the estimated aberrations and to deliver digital data representative of those shapes.
One or other of the above systems can advantageously further include a unit for fabricating a lens from digital data supplied to it by the calculator unit. This provides a complete system for fabricating a personalized contact lens, which system can be fully automated.
Other features and advantages of the invention will become apparent on reading the following description of particular embodiments of the invention, provided by way of non-limiting example.
The description refers to the accompanying drawings.